Patterned gradient polymer film and method

ABSTRACT

The present disclosure generally relates to patterned gradient polymer films and methods for making the same, and more particularly to patterned gradient optical films that have regions that include variations in optical properties such as refractive index, haze, transmission, clarity, or a combination thereof. The variation in optical properties can occur across a transverse plane of the film as well as through a thickness direction of the film.

RELATED APPLICATIONS

This application is related to U.S. Publications 2012/0021134;2012/0201977; U.S. Pat. Nos. 8,964,146; 8,891,038; 9,158,155; 9,063,293;9,279,918; and PCT Publications WO2010/120971; WO2010/1210019;WO2010/120468; WO2011/050254, the disclosure of which is incorporated byreference in their entirety herein.

BACKGROUND

Optical systems, such as retroreflecting or display systems, utilize oneor more optical layers for managing incident light. Often, the opticallayers are required to have a desired optical transmittance, opticalhaze, optical clarity, or index of refraction. In many applications, anair layer and a diffuser layer are incorporated into the optical system.Typically, the air layer supports total internal reflection and thediffuser layer provides optical diffusion.

Articles having a structure of nanometer sized pores or voids can beuseful for applications based on optical, physical, or mechanicalproperties provided by their nanovoided composition. For example, ananovoided article includes a polymeric solid network or matrix that atleast partially surrounds pores or voids. The pores or voids are oftenfilled with a gas such as air. The dimensions of the pores or voids in ananovoided article can generally be described as having an averageeffective diameter which can range from about 1 nanometer to about 1000nanometers. The International Union of Pure and Applied Chemistry(IUPAC) have provided three size categories of nanoporous materials:micropores with voids less than 2 nm, mesopores with voids between 2 nmand 50 nm, and macropores with voids greater than 50 nm. Each of thedifferent size categories can provide unique properties to a nanovoidedarticle.

SUMMARY

The present disclosure generally relates to patterned gradient polymerfilms and methods for making the same, and more particularly topatterned gradient optical films that have regions that includevariations in refractive index, haze, transmission, clarity, or acombination thereof. In one aspect, the present disclosure provides agradient polymer film that includes a binder and a plurality ofnanovoids, wherein a local volume fraction of the plurality of nanovoidsvaries across a transverse plane of the gradient polymer film.

In another aspect, the present disclosure provides a gradient polymerfilm that includes a binder and a plurality of nanovoids, wherein afirst local volume fraction of the plurality of nanovoids proximate afirst region of the gradient polymer film is greater than a second localvolume fraction of the plurality of nanovoids proximate a second regionadjacent the first region, along a transverse plane of the gradientpolymer film.

In yet another aspect, the present disclosure provides an opticalconstruction that includes a substrate and a gradient polymer filmdisposed on the substrate. Further, the gradient polymer film includes abinder and a plurality of nanovoids, wherein a local volume fraction ofthe plurality of nanovoids varies across a transverse plane of thegradient polymer film. Still further, the substrate includes at leastone of a release liner, an adhesive, a volume diffuser, a surfacediffuser, a diffractive diffuser, a refractive diffuser, aretroreflector, an absorbing polarizer, a reflective polarizer, a fiberpolarizer, a cholesteric polarizer, a multilayer polarizer, a wire gridpolarizer, a partial reflector, a volume reflector, a multilayer polymerreflector, a metal reflector, a metal/dielectric multilayer reflector, afiber, a lens, a microstructure, a solid light guide, or a hollow lightguide.

In yet another aspect, the present disclosure provides an opticalconstruction that includes a substrate and a gradient polymer filmdisposed on the substrate. Further, the gradient polymer film includes abinder and a plurality of nanovoids, wherein a first local volumefraction of the plurality of nanovoids proximate a first region of thegradient polymer film is greater than a second local volume fraction ofthe plurality of nanovoids proximate a second region adjacent the firstregion, along a transverse plane of the gradient polymer film. Stillfurther, the substrate includes at least one of a release liner, anadhesive, a volume diffuser, a surface diffuser, a diffractive diffuser,a refractive diffuser, a retroreflector, an absorbing polarizer, areflective polarizer, a fiber polarizer, a cholesteric polarizer, amultilayer polarizer, a wire grid polarizer, a partial reflector, avolume reflector, a multilayer polymer reflector, a metal reflector, ametal/dielectric multilayer reflector, a fiber, a lens, amicrostructure, a solid light guide, or a hollow light guide.

In yet another aspect, the present disclosure provides a process for agradient polymer film that includes disposing a solution on a substrateto form a coating, the coating including a polymerizable binder and asolvent; selectively polymerizing a first portion of the coating to forman insoluble polymer matrix in the solvent; removing a major portion ofthe solvent from the coating; and polymerizing a second portion of thecoating adjacent the first portion.

The above summary is not intended to describe each disclosed embodimentor every implementation of the present disclosure. The figures and thedetailed description below more particularly exemplify illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,where like reference numerals designate like elements, and wherein:

FIG. 1A is a schematic side-view of a gradient optical film;

FIGS. 1B-1I are schematic top views of gradient optical films;

FIG. 2 is a schematic side-view of an optical construction;

FIG. 3 is a schematic side-view of an optical construction;

FIG. 4 is a schematic side-view of an optical construction;

FIG. 5 is a process schematic;

FIG. 6A is a graph of Amps vs % T;

FIG. 6B is a graph of Amps vs % H;

FIG. 6C is a graph of Volts vs downweb position;

FIG. 6D is a graph of % T and % H vs downweb position;

FIG. 6E is a graph of Volts vs downweb position;

FIG. 6F is a graph of % T and % H vs downweb position;

FIG. 7A is a graph of Volts vs downweb position;

FIG. 7B is a graph of refractive index vs downweb position;

FIG. 8 is a schematic cross-section of a patterned retroreflector; and

FIG. 9 is a schematic cross-section of a patterned light guide.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

This disclosure generally relates to polymer films, in particularoptical films that exhibit some low-index-like optical properties, orotherwise interact with the transmission, scattering, absorption,refraction or reflection of light; however, it is to be understood thatthe polymer films can instead interact non-optically with theenvironment, as a result of the structure generated in the film, asdescribed elsewhere. In one particular embodiment, the optical films canexhibit low-index-like optical properties that vary along a transverseplane of the optical films, that is, gradient optical films. Thetransverse plane of a film can be described as a plane that is parallelto at least one of the surfaces of the film. Some disclosed gradientoptical films exhibit a local porosity that varies along a transverseplane of the gradient optical films. In some cases, the optical filmscan exhibit optical properties or local porosity that can also varythrough a thickness direction of the optical film. Generally, the localporosity may be described by a local void volume fraction, or as a localpore size distribution, or by both local void volume fraction, and localpore size distribution.

The disclosure also describes articles and methods to produce films withgradient optical properties/porosity within the film. These films arecharacterized by having continuous crossweb, downweb, or combinedgradients of optical properties such as transmission, haze, clarity,refractive index, etc. The gradient pattern can be created by, forexample, optically patterning a porous layer made by the describedprocess, with temporal or spatial control of the curing conditions suchas power to UV LEDs, shadow masks, controlled UV absorption, controlleddrying, or the like, or combinations thereof. The disclosed gradientfilms can be used in applications including, for example, light guidevariable extractors including solid light guide extractors, hollow (air)guide extractors, fibers and the like; gradient diffuser films (that is,varying haze, clarity, or transmission) useful, for example, for defectand/or bulb hiding, particularly in backlit displays; variablediffusers; variable absorbers; variable reflectors including enhancedspecular reflectors (ESR) for daylighting; and the like.

Some portions of the disclosed gradient optical films can have a lowoptical haze and a low effective index of refraction, such as an opticalhaze of less than about 5% and an effective index of refraction that isless than about 1.35, that can vary across a transverse plane of theoptical film. Some portions of the disclosed gradient optical films canhave a high optical haze, such as an optical haze of greater than about50%, and/or high diffuse optical reflectance while manifesting somelow-index-like optical properties, such as, for example, the ability tosupport total internal reflection or enhance internal reflection, thatcan also vary across a transverse plane of the optical film. In somecases, the disclosed gradient optical films can be incorporated invarious optical or display systems such as, for example, a generallighting system, a liquid crystal display system, or a retro-reflectingoptical system to improve system durability, reduce manufacturing cost,and reduce the overall thickness of the system while improving,maintaining or substantially maintaining at least some of the systemoptical properties such as, for example, the retro-reflectivity of thesystem or the on-axis brightness and contrast of an image displayed bythe system.

In one particular embodiment, the gradient optical films disclosedherein include variations in the properties of the optical film along atransverse plane (that is, the “x” and/or “y” directions that aremutually perpendicular to the “z”, or thickness, direction) of the film.U.S. Pat. No. 9,279,918 and U.S. Publication 2012/0201977 are generallydirected toward variations in the properties in a thickness direction(that is, the “z” direction) of the optical film. It is to be understoodthat the techniques used for making the “z” direction gradients can beused concurrently with the techniques for “x” and/or “y” directiongradients, and as such, gradient optical films can be fabricated thatinclude a variation in one, two, or all three of the mutually orthogonaldirections.

The gradient optical films typically include a plurality of nanovoids,interconnected voids, or generally a network of voids dispersed in abinder. At least some of the voids in the plurality or network areconnected to one another via hollow tunnels or hollow tunnel-likepassages. The voids are not necessarily free of all matter and/orparticulates. For example, in some cases, a void may include one or moresmall fiber- or string-like objects that include, for example, a binderand/or nano-particles. In some cases, a void may include particles orparticle agglomerates that may be attached to the binder, or may beloose within the void. Some disclosed gradient optical films includemultiple pluralities of interconnected voids or multiple networks ofvoids where the voids in each plurality or network are interconnected.In some cases, in addition to multiple pluralities of interconnectedvoids, the disclosed gradient optical films include a plurality ofclosed or unconnected voids meaning that the voids are not connected toother voids via tunnels.

In some cases, the gradient optical films can improve the durability ofportions of similar optical films that do not have a gradient structure.In some cases, portions of one surface of the gradient optical film mayresist abrasion due to, for example, a densified surface or a toughenedsurface in one region of the film surface. In some cases, the gradientoptical films may exhibit improved environmental stability, since asealed or a densified surface may prevent contaminants from entering theinterior of the gradient optical film. In some cases, a sealed ordensified surface may enhance cleanliness of the gradient optical films,since particles entrained within interior pores may become trapped suchthat mechanical forces may be unable to remove them.

In one particular embodiment, the gradient optical films can include aplurality of interconnected voids or a network of voids, such asnanovoids, having a local volume fraction or a local pore-sizedistribution that varies along a transverse plane of the gradientoptical film. As used herein, “local volume fraction” means the volumefraction of a component (for example, the plurality of interconnectedvoids, or nanovoids) measured on a local scale, and “local pore-sizedistribution” means the pore-size distribution of a component (forexample, the size distribution of the nanovoids or interconnected voids)measured on a local scale. In one particular embodiment, that is, thethickness gradients described elsewhere, the local scale can be, forexample, in a region less than about 10%, or less than about 5%, or lessthan about 3%, or less than about 1% of the total thickness of thegradient optical film. In one particular embodiment, that is, thegradients along the transverse plane described herein, the local scalecan be, for example, in a region less than about 10%, or less than about5%, or less than about 3%, or less than about 1% of the smaller of thewidth or the length of the gradient optical film.

As used herein, the local volume fraction of nanovoids and the localpore-size distribution of nanovoids are collectively referred to as a“local morphology” of the gradient film. Generally, it is a change inthe local morphology of the gradient film that produces the desiredoptical, physical (for example, thermal, electrical, acoustic,transport, surface energy), or mechanical property. In some cases, thelocal volume fraction of nanovoids can remain constant along thetransverse plane, and the local pore-size distribution of nanovoids canvary along the transverse plane. In some cases, the local volumefraction of nanovoids can vary along the transverse plane, and the localpore-size distribution of nanovoids can remain constant along thetransverse plane. In some cases, the local volume fraction of nanovoidscan vary along the transverse plane, and the local pore-sizedistribution of nanovoids can also vary along the transverse plane. In asimilar manner, each of the local volume fraction of nanovoids and thelocal pore-size distribution of nanovoids can either vary or remainconstant throughout the thickness (or “z” direction), as describedelsewhere.

In one particular embodiment, the local volume fraction can vary acrossthe transverse plane of the gradient optical film, such that the localvolume fraction proximate a first region of the film can be greater orless than the local volume fraction proximate a second region of thefilm adjacent the first region along the transverse plane of thegradient optical film. The bulk volume fraction of interconnected voidsis the ratio of the volume of voids in the optical film to the totalvolume of the optical film; in a similar manner, the bulk pore-sizedistribution is an average of the pore-size distribution taken over thetotal volume of the optical film.

In some cases, the local volume fraction can have very few nanovoids,and the film can be said to be essentially void free in that region ofthe film. In some cases, the local volume fraction can vary in acontinuous manner along a transverse plane of the film, such as either amonotonic increase or decrease in the local volume fraction along thetransverse plane of the gradient optical film. In some cases, the localvolume fraction can go through a local maximum or a local minimum acrossa transverse plane of the gradient optical film. In some cases, thelocal volume fraction can vary in a discontinuous manner alongtransverse plane of the gradient optical film, for example, astep-change in the local volume fraction of interconnected voids or thelocal pore-size distribution, or both.

Control of the local morphology can be useful in several applicationsincluding, for example, when a material is coated on a surface of thegradient optical film. In some cases, the coated material may include asolvent or other high mobility component such as, for example, a lowmolecular weight curable material, which can penetrate theinterconnected voids of the gradient optical film. In some cases, thecoated material may include a thermoplastic solid or a gelled material,such as a transfer adhesive or a pressure sensitive adhesive (PSA) that,upon thermal cycling or aging, can penetrate into the porous structureof interconnected voids. Penetration of a material into theinterconnected voids of the gradient optical film can alter propertiesof the film, including, for example, increasing the refractive index inthe penetration region.

In one particular embodiment, a change in the local morphology canprovide control over this penetration proximate one region of thegradient optical film, while maintaining a desired local volume fractionof the interconnected voids proximate an adjacent region of the gradientoptical film. In some cases, the local volume fraction proximate oneregion of the gradient optical film can be lower than the bulk volumefraction and also lower than the local volume fraction proximate anadjacent region of the gradient optical film. In some cases, the localvolume fraction can be decreased so that only limited infusion can takeplace. Limited infusion of material to form a gradient optical film canbe useful, for example, to strengthen a surface of a fragile opticalfilm that has a high bulk volume fraction of interconnected voids. Insome cases, a lower volume fraction of interconnected voids in agradient optical film can improve the structural integrity, that is, thedurability of the optical film.

In some cases, the local volume fraction can be decreased to near zerolocal volume fraction of interconnected voids, effectively sealing afirst region of the surface. Control of the local morphology can includetechniques such as, for example, inhibiting or promoting the rate andextent of cure on one or more regions of the gradient optical film,infusion of a material to at least partially fill a portion of thevoids, and the like. In general, control over the local morphology canbe accomplished by techniques described elsewhere, including, forexample, in U.S. Publication 2012/0201977.

Some disclosed gradient optical films support total internal reflection(TIR) or enhanced internal reflection (EIR) by virtue of including aplurality of voids. When light that travels in an optically clearnon-porous medium is incident on a stratum possessing high porosity, thereflectivity of the incident light is much higher at oblique angles thanat normal incidence. In the case of no or low haze voided films, thereflectivity at oblique angles greater than the critical angle is closeto about 100%. In such cases, the incident light undergoes totalinternal reflection (TIR). In the case of high haze voided films, theoblique angle reflectivity can be close to 100% over a similar range ofincident angles even though the light may not undergo TIR. This enhancedreflectivity for high haze films is similar to TIR and is designated asEnhanced Internal Reflectivity (EIR). As used herein, by a porous orvoided gradient optical film enhancing internal reflection (EIR), it ismeant that the reflectance at the boundary of the voided and non-voidedstrata of the film or film laminate is greater with the voids thanwithout the voids.

The voids in the disclosed gradient optical films have an index ofrefraction n_(v) and a permittivity ε_(v), where n_(v) ²=ε_(v), and thebinder has an index of refraction n_(b) and a permittivity ε_(b), wheren_(b) ²=ε_(b). In general, the interaction of a gradient optical filmwith light, such as light that is incident on, or propagates in, thegradient optical film, depends on a number of film characteristics suchas, for example, the film thickness, the binder index, the void or poreindex, the pore shape and size, the spatial distribution of the pores,and the wavelength of light. In some cases, light that is incident on orpropagates within the gradient optical film, “sees” or “experiences” aneffective permittivity ε_(eff) and an effective index n_(eff), wheren_(eff) can be expressed in terms of the void index n_(v), the binderindex n_(b), and the void porosity or volume fraction “f”. In suchcases, the gradient optical film is sufficiently thick and the voids aresufficiently small so that light cannot resolve the shape and featuresof a single or isolated void. In such cases, the size of at least amajority of the voids, such as at least 60% or 70% or 80% or 90% of thevoids, is not greater than about λ/5, or not greater than about λ/6, ornot greater than about λ/8, or not greater than about λ/10, or notgreater than about λ/20, where λ is the wavelength of light.

In some cases, light that is incident on a disclosed gradient opticalfilm is a visible light meaning that the wavelength of the light is inthe visible range of the electromagnetic spectrum. In such cases, thevisible light has a wavelength that is in a range from about 380 nm toabout 750 nm, or from about 400 nm to about 700 nm, or from about 420 nmto about 680 nm. In such cases, the gradient optical film has aneffective index of refraction and includes a plurality of voids if thesize of at least a majority of the voids, such as at least 60% or 70% or80% or 90% of the voids, is not greater than about 70 nm, or not greaterthan about 60 nm, or not greater than about 50 nm, or not greater thanabout 40 nm, or not greater than about 30 nm, or not greater than about20 nm, or not greater than about 10 nm.

In some cases, the disclosed gradient optical films are sufficientlythick so that the gradient optical film can reasonably have an effectiveindex that can be expressed in terms of the indices of refraction of thevoids and the binder, and the void or pore volume fraction or porosity.In such cases, the thickness of the gradient optical film is not lessthan about 100 nm, or not less than about 200 nm, or not less than about500 nm, or not less than about 700 nm, or not less than about 1,000 nm.

When the voids in a disclosed gradient optical film are sufficientlysmall and the gradient optical film is sufficiently thick, the gradientoptical film has an effective permittivity ε_(eff) that can be expressedas:ε_(eff) =fε _(v)+(1−f)ε_(b)  (1)

In such cases, the effective index nay of the gradient optical film canbe expressed as:n _(eff) ² =fn _(v) ²+(1−f)n _(b) ²  (2)

In some cases, such as when the difference between the indices ofrefraction of the pores and the binder is sufficiently small, theeffective index of the gradient optical film can be approximated by thefollowing expression:n _(eff) =fn _(v)+(1−f)n _(b)  (3)

In such cases, the effective index of the gradient optical film is thevolume weighted average of the indices of refraction of the voids andthe binder. For example, a gradient optical film that has a void volumefraction of about 50% and a binder that has an index of refraction ofabout 1.5, has an effective index of about 1.25.

FIG. 1A is a schematic side-view of a gradient optical film 300A thatincludes a network of voids or plurality of interconnected voids 320 anda plurality of particles 340 dispersed within a binder 310. Gradientoptical film 300A has a porous interior by virtue of the presence ofnetwork of voids 320 within the gradient optical film. In general, thegradient optical film can include one or more networks of interconnectedpores or voids. For example, network of voids 320 can be regarded toinclude interconnected voids or pores 320A-320C.

In some cases, a local morphology, for example a first local volumefraction of interconnected voids 370A and a second volume fraction ofinterconnected voids 375A, can vary along a thickness ti direction (alsoreferred to as the “z” direction) within gradient optical film 300A. InFIG. 1A, for example, first local volume fraction of interconnectedvoids 370A has been depicted as being greater than second volumefraction of interconnected voids 375A. The local volume fraction ofinterconnected voids, and pore-size distribution, can vary along thethickness direction in several ways, for example, as described in U.S.Publication 2012/0201977 and U.S. Pat. No. 9,279,918.

In some cases, a local volume fraction of interconnected voids, forexample a third local volume fraction of interconnected voids 372, afourth local volume fraction of interconnected voids 374, and a fifthlocal volume fraction of interconnected voids 376, can vary along thedirection of a transverse plane “L” (that is, generally along the “x”and/or “y” direction) within gradient optical film 300A. In FIG. 1A, forexample, fifth local volume fraction of interconnected voids 376 hasbeen depicted as being greater than either the third local volumefraction of interconnected voids 372 or the fourth local volume fractionof interconnected voids 374. The local volume fraction of interconnectedvoids, and void size distribution, can also vary along the thicknessdirection in several ways, as described elsewhere. In some cases, thegradient optical film is a porous film meaning that the network of voids320 forms one or more passages between first and second major surfaces330 and 332, respectively. In some cases, the local volume fraction ofinterconnected voids can vary along any combination of the “x”, “y”, and“z” directions.

The network of voids can be regarded to include a plurality ofinterconnected voids. Some of the voids can be at a surface of thegradient optical film and can be regarded to be surface voids. Forexample, in the exemplary gradient optical film 300A, voids 320D and320E are at a second major surface 332 of the gradient optical film andcan be regarded as surface voids 320D and 320E, and voids 320F and 320Gare at a first major surface 330 of the gradient optical film and can beregarded as surface voids 320F and 320G. Some of the voids, such as forexample voids 320B and 320C, are within the interior of the gradientoptical film and away from the exterior surfaces of the gradient opticalfilm and can be regarded as interior voids 320B and 320C, even though aninterior void can be connected to a major surface via, for example,other voids.

Voids 320 have a size d₁ that can generally be controlled by choosingsuitable composition and fabrication techniques, such as the variouscoating, drying and curing conditions. In general, d₁ can be any desiredvalue in any desired range of values. For example, in some cases, atleast a majority of the voids, such as at least 60% or 70% or 80% or 90%or 95% of the voids, have a size that is in a desired range. Forexample, in some cases, at least a majority of the voids, such as atleast 60% or 70% or 80% or 90% or 95% of the voids, have a size that isnot greater than about 10 microns, or not greater than about 7 microns,or not greater than about 5 microns, or not greater than about 4microns, or not greater than about 3 microns, or not greater than about2 microns, or not greater than about 1 micron, or not greater than about0.7 microns, or not greater than about 0.5 microns.

In some cases, plurality of interconnected voids 320 has an average voidor pore size that is not greater than about 5 microns, or not greaterthan about 4 microns, or not greater than about 3 microns, or notgreater than about 2 microns, or not greater than about 1 micron, or notgreater than about 0.7 microns, or not greater than about 0.5 microns.

In some cases, some of the voids can be sufficiently small so that theirprimary optical effect is to reduce the effective index, while someother voids can reduce the effective index and scatter light, whilestill some other voids can be sufficiently large so that their primaryoptical effect is to scatter light.

Particles 340 have a size d₂ that can be any desired value in anydesired range of values. For example, in some cases at least a majorityof the particles, such as at least 60% or 70% or 80% or 90% or 95% ofthe particles, have a size that is in a desired range. For example, insome cases, at least a majority of the particles, such as at least 60%or 70% or 80% or 90% or 95% of the particles, have a size that is notgreater than about 5 microns, or not greater than about 3 microns, ornot greater than about 2 microns, or not greater than about 1 micron, ornot greater than about 700 nm, or not greater than about 500 nm, or notgreater than about 200 nm, or not greater than about 100 nm, or notgreater than about 50 nm, or even not greater than about 20 nm.

In some cases, plurality of particles 340 has an average particle sizethat is not greater than about 5 microns, or not greater than about 3microns, or not greater than about 2 microns, or not greater than about1 micron, or not greater than about 700 nm, or not greater than about500 nm, or not greater than about 200 nm, or not greater than about 100nm, or not greater than about 50 nm.

In some cases, some of the particles can be sufficiently small so thatthey primarily affect the effective index, while some other particlescan affect the effective index and scatter light, while still some otherparticles can be sufficiently large so that their primary optical effectis to scatter light.

In some cases, d₁ and/or d₂ are sufficiently small so that the primaryoptical effect of the voids and the particles is to affect the effectiveindex of gradient optical film 300A. For example, in such cases, d₁and/or d₂ are not greater than about λ/5, or not greater than about λ/6,or not greater than about λ/8, or not greater than about λ/10, or notgreater than about λ/20, where X is the wavelength of light. As anotherexample, in such cases, d₁ and d₂ are not greater than about 70 nm, ornot greater than about 60 nm, or not greater than about 50 nm, or notgreater than about 40 nm, or not greater than about 30 nm, or notgreater than about 20 nm, or not greater than about 10 nm. In suchcases, the voids and the particles may also scatter light, but theprimary optical effect of the voids and the particles is to define aneffective medium in the gradient optical film that has an effectiveindex. The effective index depends, in part, on the indices ofrefraction of the voids, the binder, and the particles. In some cases,the effective index is a reduced effective index, meaning that theeffective index is less than the index of the binder and the index ofthe particles.

In cases where the primary optical effect of the voids and/or theparticles is to affect the index, d₁ and d₂ are sufficiently small sothat a substantial fraction, such as at least about 60%, or at leastabout 70%, or at least about 80%, or at least about 90%, or at leastabout 95% of voids 320 and particles 340 have the primary optical effectof reducing the effective index. In such cases, a substantial fraction,such as at least about 60%, or at least about 70%, or at least about80%, or at least about 90%, or at least about 95% the voids and/or theparticles, have a size that is in a range from about 1 nm to about 200nm, or from about 1 nm to about 150 nm, or from about 1 nm to about 100nm, or from about 1 nm to about 50 nm, or from about 1 nm to about 20nm.

In some cases, the index of refraction n₁ of particles 340 can besufficiently close to the index n_(b) of binder 310, so that theeffective index does not depend, or depends very little, on the index ofrefraction of the particles. In such cases, the difference between n₁and n_(b) is not greater than about 0.01, or not greater than about0.007, or not greater than about 0.005, or not greater than about 0.003,or not greater than about 0.002, or not greater than about 0.001. Insome cases, particles 340 are sufficiently small and their index issufficiently close to the index of the binder, that the particles do notprimarily scatter light or affect the index. In such cases, the primaryeffect of the particles can, for example, be to enhance the strength ofgradient optical film 300A. In some cases, particles 340 can enhance theprocess of making the gradient optical film, although gradient opticalfilm 300A can be made with no particles.

In cases where the primary optical effect of network of voids 320 andparticles 340 is to affect the effective index and not to, for example,scatter light, the optical haze of gradient optical film 300A that isdue to voids 320 and particles 340 is not greater than about 5%, or notgreater than about 4%, or not greater than about 3.5%, or not greaterthan about 4%, or not greater than about 3%, or not greater than about2.5%, or not greater than about 2%, or not greater than about 1.5%, ornot greater than about 1%. In such cases, the effective index of theeffective medium of the gradient optical film is not greater than about1.35, or not greater than about 1.3, or not greater than about 1.25, ornot greater than about 1.2, or not greater than about 1.15, or notgreater than about 1.1, or not greater than about 1.05.

In cases where gradient optical film 300A can reasonably have a reducedeffective index, the thickness of the gradient optical film is not lessthan about 100 nm, or not less than about 200 nm, or not less than about500 nm, or not less than about 700 nm, or not less than about 1,000 nm,or not less than about 1500 nm, or not less than about 2000 nm.

In some cases, d₁ and/or d₂ are sufficiently large so that their primaryoptical effect is to scatter light and produce optical haze. In suchcases, d₁ and/or d₂ are not less than about 200 nm, or not less thanabout 300 nm, or not less than about 400 nm, or not less than about 500nm, or not less than about 600 nm, or not less than about 700 nm, or notless than about 800 nm, or not less than about 900 nm, or not less thanabout 1000 nm. In such cases, the voids and the particles may alsoaffect the index, but often, their primarily optical effect is toscatter light. In such cases, light incident on the gradient opticalfilm can be scattered by both the voids and the particles.

Gradient optical film 300A can be used in many optical applications. Forexample, in some cases, the gradient optical film can be used to supportor promote total internal reflection (TIR) or enhance internalreflection meaning that the reflection is greater than what a materialwith index n_(b) would produce. In such cases, gradient optical film300A is sufficiently thick so that the evanescent tail of a light raythat undergoes total internal reflection at a surface of the gradientoptical film, does not optically couple, or optically couples verylittle, or even is controllably coupled, across the thickness of thegradient optical film. In such cases, the thickness ti of gradientoptical film 300A is not less than about 1 micron, or not less thanabout 1.1 micron, or not less than about 1.2 microns, or not less thanabout 1.3 microns, or not less than about 1.4 microns, or not less thanabout 1.5 microns, or not less than about 1.7 microns, or not less thanabout 2 microns. A sufficiently thick gradient optical film 300A canprevent or reduce an undesired optical coupling of the evanescent tailof an optical mode across the thickness of the gradient optical film.The TIR properties of the gradient optical film can vary in differentregions of the film, along the transverse plane, as described elsewhere.

In some cases, portions of the gradient optical film 300A have a lowoptical haze. In such cases, the optical haze of the gradient opticalfilm is not greater than about 5%, or not greater than about 4%, or notgreater than about 3.5%, or not greater than about 4%, or not greaterthan about 3%, or not greater than about 2.5%, or not greater than about2%, or not greater than about 1.5%, or not greater than about 1%. Insuch cases, the gradient optical film can have a reduced effective indexthat is not greater than about 1.35, or not greater than about 1.3, ornot greater than about 1.2, or not greater than about 1.15, or notgreater than about 1.1, or not greater than about 1.05. For lightnormally incident on gradient optical film 300A, optical haze, as usedherein, is defined as the ratio of the transmitted light that deviatesfrom the normal direction by more than 4 degrees to the totaltransmitted light. Haze values disclosed herein were measured using aHaze-Gard Plus haze meter (BYK-Gardner, Silver Springs, Md.) accordingto the procedure described in ASTM D1003. The haze properties of thegradient optical film can vary in different regions of the film, alongthe transverse plane, as described elsewhere.

In some cases, portions of the gradient optical film 300A have a highoptical haze. In such cases, the haze of the gradient optical film isnot less than about 40%, or not less than about 50%, or not less thanabout 60%, or not less than about 70%, or not less than about 80%, ornot less than about 90%, or not less than about 95%. In some cases,gradient optical film 300A can have an intermediate optical haze, forexample, between about 5% and about 50% optical haze.

In some cases, portions of the gradient optical film 300A have a highdiffuse optical reflectance. In such cases, the diffuse opticalreflectance of the gradient optical film is not less than about 30%, ornot less than about 40%, or not less than about 50%, or not less thanabout 60%. The diffuse optical reflectance of the gradient optical filmcan vary in different regions of the film, along the transverse plane,as described elsewhere.

In some cases, portions of the gradient optical film 300A have a highoptical clarity. For light normally incident on gradient optical film300A, optical clarity, as used herein, refers to the ratio(T₂−T₁)/(T₁+T₂), where T₁ is the transmitted light that deviates fromthe normal direction between 1.6 and 2 degrees, and T₂ is thetransmitted light that lies between zero and 0.7 degrees from the normaldirection. Clarity values disclosed herein were measured using aHaze-Gard Plus haze meter from BYK-Gardner. In the cases where gradientoptical film 300A has a high optical clarity, the clarity is not lessthan about 40%, or not less than about 50%, or not less than about 60%,or not less than about 70%, or not less than about 80%, or not less thanabout 90%, or not less than about 95%. The optical clarity of thegradient optical film can vary in different regions of the film, alongthe transverse plane, as described elsewhere.

In some cases, portions of the gradient optical film 300A have a lowoptical clarity. In such cases, the optical clarity of the gradientoptical film is not greater than about 40%, or not greater than about20%, or not greater than about 10%, or not greater than about 7%, or notgreater than about 5%, or not greater than about 4%, or not greater thanabout 3%, or not greater than about 2%, or not greater than about 1%.

In general, gradient optical film can have any porosity, pore-sizedistribution, or void volume fraction that may be desirable in anapplication. In some cases, the volume fraction of plurality of voids320 in gradient optical film 300A is not less than about 20%, or notless than about 30%, or not less than about 40%, or not less than about50%, or not less than about 60%, or not less than about 70%, or not lessthan about 80%, or not less than about 90%.

In some cases, portions of the gradient optical film can manifest somelow-index properties, even if the gradient optical film has a highoptical haze and/or diffuse reflectance. For example, in such cases, theportions of the gradient optical film can support TIR at angles thatcorrespond to an index that is smaller than the index n_(b) of binder310.

In the exemplary gradient optical film 300A, particles 340, such asparticles 340A and 340B, are solid particles. In some cases, gradientoptical film 300A may additionally or alternatively include a pluralityof hollow or porous particles 350.

Particles 340 can be any type particles that may be desirable in anapplication. For example, particles 340 can be organic or inorganicparticles. For example, particles 340 can be silica, zirconium oxide oralumina particles.

Particles 340 can have any shape that may be desirable or available inan application. For example, particles 340 can have a regular orirregular shape. For example, particles 340 can be approximatelyspherical. As another example, the particles can be elongated. In suchcases, gradient optical film 300A includes a plurality of elongatedparticles 340. In some cases, the elongated particles have an averageaspect ratio that is not less than about 1.5, or not less than about 2,or not less than about 2.5, or not less than about 3, or not less thanabout 3.5, or not less than about 4, or not less than about 4.5, or notless than about 5. In some cases, the particles can be in the form orshape of a string-of-pearls (such as Snowtex-PS particles available fromNissan Chemical, Houston, Tex.) or aggregated chains of spherical oramorphous particles, such as fumed silica.

Particles 340 may or may not be functionalized. In some cases, particles340 are not functionalized. In some cases, particles 340 arefunctionalized so that they can be dispersed in a desired solvent orbinder 310 with no, or very little, clumping. In some cases, particles340 can be further functionalized to chemically bond to binder 310. Forexample, particles 340, such as particle 340A, can be surface modifiedand have reactive functionalities or groups 360 to chemically bond tobinder 310. In such cases, at least a significant fraction of particles340 is chemically bound to the binder. In some cases, particles 340 donot have reactive functionalities to chemically bond to binder 310. Insuch cases, particles 340 can be physically bound to binder 310, orbinder 310 can encapsulate particles 340.

In some cases, some of the particles have reactive groups and others donot have reactive groups. For example in some cases, about 10% of theparticles have reactive groups and about 90% of the particles do nothave reactive groups, or about 15% of the particles have reactive groupsand about 85% of the particles do not have reactive groups, or about 20%of the particles have reactive groups and about 80% of the particles donot have reactive groups, or about 25% of the particles have reactivegroups and about 75% of the particles do not have reactive groups, orabout 30% of the particles have reactive groups and about 60% of theparticles do not have reactive groups, or about 35% of the particleshave reactive groups and about 65% of the particles do not have reactivegroups, or about 40% of the particles have reactive groups and about 60%of the particles do not have reactive groups, or about 45% of theparticles have reactive groups and about 55% of the particles do nothave reactive groups, or about 50% of the particles have reactive groupsand about 50% of the particles do not have reactive groups. In somecases, some of the particles may be functionalized with both reactiveand unreactive groups on the same particle.

The ensemble of particles may include a mixture of sizes, reactive andnon-reactive particles and different types of particles, for example,organic particles including polymeric particles such as acrylics,polycarbonates, polystyrenes, silicones and the like; or inorganicparticles such as glasses or ceramics including, for example, silica andzirconium oxide, and the like.

Binder 310 can be or include any material that may be desirable in anapplication. For example, binder 310 can be a curable material thatforms a polymer, such as a cross-linked polymer. In general, binder 310can be any polymerizable material, such as a polymerizable material thatis radiation-curable, such as a UV curable material.

Gradient optical film 300A can be produced using any method that may bedesirable in an application. In some cases, gradient optical film 300Acan be produced by the processes described in PCT PublicationWO2010/120468 and U.S. Publications 2012/0021134 and 2012/0201977, thedisclosures of which are incorporated in their entirety herein byreference.

Generally, in one process typically referred to herein as the “GEL”process, first a solution is prepared that includes a plurality ofparticles, such as nano-particles, and a polymerizable materialdissolved in a solvent, where the polymerizable material can include,for example, one or more types of monomers. Next, the polymerizablematerial is polymerized, for example by applying heat or light, to forman insoluble polymer matrix in the solvent. As the polymerizationoccurs, the solvent solubility (in the cured matrix) decreases and canphase separate from the matrix. This results in a matrix-rich networkand phase separated solvent-rich network. The solvent is subsequentlyremoved leaving pores and voids which yield the porous coating. Theextent and type of phase separation is a primary factor in determiningthe morphology and topography of the film. The final structure is alsodependent on the mechanical properties of the matrix network. Thenetwork modulus and strength should be sufficient to maintain a voidspace as the solvent is removed. The composition and extent of cure arefactors in determining the morphology.

By controlling the polymerization, drying, and cure environment, themorphology can be controlled. The process may also utilize a controlledenvironment region between the coating station and polymerizationapparatus, as described elsewhere. This region enables improved controlof the coated film composition and environment. The polymerizationapparatus can be located anywhere between the coating station and dryer.Controlling the environment during polymerization is also advantageous.The polymerized coating is subsequently dried and may be furtherpost-processed with, for example, conventional UV radiation systems tofurther cure the material. Radiation sources that could be used in thepolymerization apparatus include LEDs, UV lasers, UV lamps, and e-beam).

In some cases, after the polymerization step, the solvent may stillinclude some of the polymerizable material, although at a lowerconcentration. Next, the solvent is removed by drying or evaporating thesolution resulting in gradient optical film 300A that includes anetwork, or a plurality, of voids 320 dispersed in polymer binder 310.The gradient optical film further includes plurality of particles 340dispersed in the polymer. The particles are bound to the binder, wherethe bonding can be physical or chemical, or be encapsulated by thebinder.

Gradient optical film 300A can have other materials in addition tobinder 310 and particles 340. For example, gradient optical film 300Acan include one or more additives, such as for example, coupling agents,to help wet the surface of a substrate, not expressly shown in FIG. 1,on which the gradient optical film is formed. As another example,gradient optical film 300A can include one or more colorants, such acarbon black, for imparting a color, such as the black color, to thegradient optical film. Other exemplary materials in gradient opticalfilm 300A include initiators, such as one or more photo-initiators;anti-stats; adhesion promoters; surfactants; UV absorbers; releaseagents; or others, as described elsewhere. In some cases, gradientoptical film 300A can include a down converting material that is capableof absorbing light and reemitting a longer wavelength light. Exemplarydown converting materials include phosphors.

In general, gradient optical film 300A can have a range of desirableporosities for any weight ratio of binder 310 to plurality of particles340. Accordingly, in general, the weight ratio can be any value that maybe desirable in an application. In some cases, the weight ratio ofbinder 310 to plurality of particles 340 is not less than about 1:2.5,or not less than about 1:2.3, or not less than about 1:2, or not lessthan about 1:1, or not less than about 1.5:1, or not less than about2:1, or not less than about 2.5:1, or not less than about 3:1, or notless than about 3.5:1, or not less than about 4:1, or not less thanabout 5:1. In some cases, the weight ratio is in a range from about1:2.3 to about 4:1.

In some cases, top major surface 332 of gradient optical film 300A canbe treated to, for example, improve the adhesion of the gradient opticalfilm to another layer. For example, the top surface can be coronatreated.

FIGS. 1B-1I are schematic top views of a gradient optical film300B-300I, respectively, according to different aspects of thedisclosure. For clarity, the numbered elements 310-360 and the sizesd₁-d₃ described for FIG. 1A are not shown in FIGS. 1B-1I; however, eachof the descriptions provided for gradient optical film 300A of FIG. 1Aalso correspond to the gradient optical film 300B-300I of FIGS. 1B-1I,respectively. It is to be understood that any of the techniques forcreating a gradient optical film that varies with thickness can also beused in conjunction with the gradient optical films that vary across thetransverse plane (parallel to the surface of a film) as shown in FIGS.1A-1I. Techniques for variation in thickness gradients are described,for example, in U.S. Publication 2012/0201977.

In one particular embodiment, gradient optical films having transverseplane variations can be generated, for example, by using a difference inthe polymerization initiator concentration or a difference in thepolymerization inhibitor concentration proximate adjacent regions. Inone particular embodiment, a shadow mask can be positioned between thelamps and the coating, such that the intensity of polymerization lightdecreases proximate adjacent regions. In one particular embodiment, theintensity of radiation can be temporally or spatially varied across thewidth of the coating, affecting the local morphology, as describedelsewhere. In one particular embodiment, a multilayer coating techniquecan be used, for example, where the regions include different ratios ofpolymeric binder to particles.

Several techniques can be used to impose the gradient structure,including, for example, techniques that modify dose; solventmodification techniques; chemical, coating and external techniques; andothers that can be envisioned to one of skill in the art. Techniquesthat modify dose include, for example, light source techniques includingtemporal modification (pulse the LEDs), LED laser writing, control ofdifferent wavelength light sources, and video image (moves with theweb); mask techniques including shadow masks, grayscale masks, printedmasks, and masks interior to a transparent roll with light sourceinside; and machine techniques including web speed variation, variationof distance or focus of light. Solvent modification techniques include,for example, temperature gradients; differential drying techniquesincluding vacuum, flow, masked drying, and saturation of gas; andsolvent coating techniques including coating in stripes of otherpatterns. Chemical techniques include, for example, patternedphotoinitiator and patterned photoinhibitor including chemicaladditives, gasses, and oxygen inhibition. Coating techniques include,for example, stripe coating and pattern overcoating. External techniquesinclude, for example, applied fields such as, for example, electric ormagnetic or the like.

In general, any desired pattern can be generated by combinations of thedescribed techniques, including, for example, indicia such as letters,words, symbols, or even pictures. The patterns can also be continuous,discontinuous, monotonic, serpentine, any smoothly varying function;stripes; varying in the machine direction, the transverse direction, orboth; gradients can form an image, logo, or text; and they can includepatterned coatings and/or perforations.

In FIG. 1B, gradient optical film 300B includes a length L and a width Wthat defines a transverse plane LW. Gradient optical film 300B furtherincludes a local morphology 390B that varies along the transverse planeLW, for example, in a monotonic manner as shown. In one particularembodiment, a first local volume fraction of interconnected voids 370Bproximate a first edge 330B of gradient optical film 300B is lower thana second local volume fraction of interconnected voids 375B proximate asecond edge 332B of gradient optical film 300B, and varies monotonicallybetween the edges. Gradient optical film 300B can be prepared using avariety of techniques, as described elsewhere.

In FIG. 1C, gradient optical film 300C includes a length L and a width Wthat defines a transverse plane LW. Gradient optical film 300C furtherincludes a local morphology 390C that varies along the transverse planeLW, for example, in a step-wise manner as shown. In one particularembodiment, a first local volume fraction of interconnected voids 370Cproximate a first edge 330C of gradient optical film 300C is lower thana second local volume fraction of interconnected voids 375C proximate asecond edge 332C of gradient optical film 300C. In some cases, forexample, shown FIG. 1C, first local volume fraction of interconnectedvoids 370C transitions sharply (that is, step-wise) to second localvolume fraction of interconnected voids 375C. In some cases, a linewidth d1 of the second volume fraction of interconnected voids 375C canbe a small percentage of the width W, for example, from about 1% toabout 5%, or to about 10%, or to about 20%, or to about 30% or more ofthe total width W. Any number of regions having the first local volumefraction of interconnected voids 370C can be formed across the width Wof the gradient optical film 300C, as apparent to those of skill in theart. Gradient optical film 300C can be prepared using a variety oftechniques, as described elsewhere.

In FIG. 1D, gradient optical film 300D includes a length L and a width Wthat defines a transverse plane LW. Gradient optical film 300D furtherincludes a local morphology 390D that varies along the transverse planeLW, for example, having a minimum local volume fraction ofinterconnected voids 377D as shown. In one particular embodiment, afirst local volume fraction of interconnected voids 370D proximate afirst edge 330D of gradient optical film 300D is approximately the sameas a second local volume fraction of interconnected voids 375D proximatea second edge 332D of gradient optical film 300D. In some cases, forexample, shown FIG. 1D, first local volume fraction of interconnectedvoids 370D transitions sharply (that is, step-wise) to minimum localvolume fraction of interconnected voids 377D. In some cases, thetransition can be abrupt, as in a step-change, or the transition can besmoothed slightly, for example, an “S” shaped transition (not shown). Insome cases, a line width d1 of the minimum volume fraction ofinterconnected voids 377D can be a small percentage of the width W, forexample, from about 1% to about 5%, or to about 10%, or to about 20%, orto about 30% or more of the width W. In some cases, the relativeposition of the minimum local volume fraction of interconnected voids377D can be located anywhere, and in multiple positions across the widthW. Gradient optical film 300D can be prepared using a variety oftechniques, as described elsewhere.

In FIG. 1E, gradient optical film includes a length L and a width W thatdefines a transverse plane LW. Gradient optical film 300E furtherincludes a local morphology 390E that varies along the transverse planeLW, for example, having a step-change local volume fraction ofinterconnected voids proximate a first and a second edge 330E, 332E, asshown. In one particular embodiment, a first local volume fraction ofinterconnected voids 370E proximate a first edge 330E of gradientoptical film 300E is approximately the same as a second local volumefraction of interconnected voids 375E proximate a second edge 332E ofgradient optical film 300E. In some cases, for example, shown FIG. 1E,first local volume fraction of interconnected voids 370E transitionssharply (that is, step-wise) to maximum local volume fraction ofinterconnected voids 377E. In some cases, each of the first and secondlocal volume fraction of interconnected voids 370E and 375E can havetransitions that are not step-wise (not shown, but similar to themonotonic variation shown in FIG. 1B). Gradient optical film 300E can beprepared using a variety of techniques, as described elsewhere.

In FIG. 1F, gradient optical film 300F includes a length L and a width Wthat defines a transverse plane LW. Gradient optical film 300F furtherincludes a local morphology 390F that varies along the transverse planeLW, for example, having a gradient minimum local volume fraction ofinterconnected voids 377F as shown. In one particular embodiment, afirst local volume fraction of interconnected voids 370F proximate afirst edge 330F of gradient optical film 300F is approximately the sameas a second local volume fraction of interconnected voids 375F proximatea second edge 332F of gradient optical film 300F. In some cases, forexample, shown FIG. 1F, first local volume fraction of interconnectedvoids 370F transitions gradually (that is, in a monotonic gradient) to aminimum local volume fraction of interconnected voids 377F, and againtransitions gradually to the second volume fraction of interconnectedvoids 375F. Gradient optical film 300F can be prepared using a varietyof techniques, as described elsewhere.

In FIG. 1G, gradient optical film 300G includes a length L and a width Wthat defines a transverse plane LW. Gradient optical film 300G furtherincludes a local morphology 390G that varies along the transverse planeLW, for example, having a pair of step-change local volume fraction ofinterconnected voids 377G, 378G, as shown. In one particular embodiment,a first local volume fraction of interconnected voids 370G proximate afirst edge 330G of gradient optical film 300G is approximately the sameas a second local volume fraction of interconnected voids 375G proximatea second edge 332G of gradient optical film 300G. In some cases, forexample, shown FIG. 1G, first local volume fraction of interconnectedvoids 370G transitions sharply (that is, step-wise) to minimum localvolume fraction of interconnected voids 377G, transitions sharply againto a maximum local volume fraction of interconnected voids 380G,transitions sharply again to a minimum local volume fraction ofinterconnected voids 378G, and finally transitions sharply yet again tothe second local volume fraction of interconnected voids 375G. In somecases, each of the local volume fraction of interconnected voids canhave transitions that are not step-wise (not shown, but similar to themonotonic variation shown in FIG. 1B). Gradient optical film 300G can beprepared using a variety of techniques, as described elsewhere.

In FIG. 1H, gradient optical film 300H includes a length L and a width Wthat defines a transverse plane LW. Gradient optical film 300H furtherincludes a local morphology 390H that varies along the transverse planeLW, for example, having a step-change local volume fraction ofinterconnected voids 380H, 382H that change along the length L of thegradient optical film 300H, as shown. In one particular embodiment, afirst local volume fraction of interconnected voids 380H isperpendicular to both a first edge 330H and a second edge 332H ofgradient optical film 300G, and a second local volume fraction ofinterconnected voids 382H is also perpendicular to the first and secondedges 330H, 332H of gradient optical film 300G. In some cases, forexample, shown FIG. 1H, first local volume fraction of interconnectedvoids 380H transitions sharply (that is, step-wise) to minimum localvolume fraction of interconnected voids 382H, and continues in a likefashion down the length L of the gradient optical film. In some cases,each of the local volume fraction of interconnected voids can havetransitions that are not step-wise (not shown, but similar to themonotonic variation shown in FIG. 1B). Gradient optical film 300H can beprepared using a variety of techniques, as described elsewhere.

In FIG. 1I, gradient optical film 300I includes a length L and a width Wthat defines a transverse plane LW. Gradient optical film 300I furtherincludes a local morphology 390I that varies along the transverse planeLW, for example, having a step-change local volume fraction ofinterconnected voids 380I, 382I that change in a checkerboard fashion,as shown. It is to be understood that any desired pattern can be formedacross the transverse plane, including, for example, geometric shapes,words, indicia, images, and the like. In one particular embodiment, forexample shown FIG. 1I, first local volume fraction of interconnectedvoids 380I transitions sharply (that is, step-wise) to minimum localvolume fraction of interconnected voids 382I, and continues in a likefashion across the transverse plane LW of the gradient optical film. Insome cases, each of the local volume fraction of interconnected voidscan have transitions that are not step-wise (not shown, but similar tothe monotonic variation shown in FIG. 1B). Gradient optical film 300Ican be prepared using a variety of techniques, as described elsewhere.

FIG. 2 is a schematic side-view of an optical construction 600 thatincludes a gradient optical film 630 disposed on a substrate 610. Insome cases, substrate 610 is a release liner that provides atransferable gradient optical film 630, meaning that, for example, theexposed top major surface 632 of the gradient optical film 630 may beplaced in contact with a substrate or surface and the release liner maythereafter be stripped away from the gradient optical film to expose abottom major surface 634 of the gradient optical film that can, forexample, be bonded to another substrate or surface. The release forcefor releasing low index layer 630 from a release liner 610 is generallyless than about 200 g-force/inch, or less than about 150 g-force/inch,or less than about 100 g-force/inch, or less than about 75 g-force/inch,or less than about 50 g-force/inch.

Gradient optical film 630 can be similar to any gradient optical filmdisclosed herein. For example, gradient optical film 630 can be similarto one of gradient optical films 300A-300I. In some cases, gradientoptical film 630 can include multiple layers, where one or more layersis similar to one of gradient optical films 300A-300I, one or morelayers includes a “z” gradient film as described elsewhere, or one ormore layers includes a non-gradient film, or a combination of gradientfilms and non-gradient films. In some cases, one of gradient opticalfilms 300A-300I may be coated directly onto substrate 610. In somecases, one of gradient optical films 300A-300I may be first formed andthereafter transferred onto substrate 610. Substrate 610 can betranslucent, transparent, or opaque.

Substrate 610 can be or include any material that may be suitable in anapplication, such as a dielectric, a semiconductor, or a conductor (suchas a metal). For example, substrate 610 can include or be made of glassand polymers such as polyethylene terapthalate (PET), polycarbonates,and acrylics. In some cases, the substrate 610 can include a polarizersuch as a reflective polarizer, an absorbing polarizer, a wire-gridpolarizer, or a fiber polarizer. In some case, the substrate 610 caninclude multiple layers, such as a multilayer optical film including,for example, multilayer reflecting films and multilayer polarizingfilms. In some cases, the substrate 610 can include a structuredsurface, such as a surface having a plurality of microstructuresincluding, for example, vee-grooves such as a Brightness Enhancing Film(BEF), cube-corners, such as a retroreflector, or other microstructuresas known in the art. In some cases, the substrate 610 can includefurther coatings on a major surface such as, for example, a primercoating or an adhesive coating.

As used herein, a fiber polarizer includes a plurality of substantiallyparallel fibers that form one or more layers of fibers embedded within abinder with at least one of the binder and the fibers including abirefringent material. The substantially parallel fibers define atransmission axis and a reflection axis. The fiber polarizersubstantially transmits incident light that is polarized parallel to thetransmission axis and substantially reflects incident light that ispolarized parallel to the reflection axis. Examples of fiber polarizersare described in, for example, U.S. Pat. Nos. 7,599,592 and 7,526,164,the entireties of which are incorporated herein by reference.

In some cases, the substrate 610 can include a partial reflector. Apartial reflector is an optical element or a collection of opticalelements which reflect at least 30% of incident light while transmittingthe remainder, minus absorption losses. Suitable partial reflectorsinclude, for example, foams, polarizing and non-polarizing multilayeroptical films, microreplictated structures (for example BEF), polarizedand non-polarized blends, wire grid polarizers, partially transmissivemetals such as silver or nickel, metal/dielectric stacks such as silverand indium tin oxide, and asymmetric optical films. Asymmetric opticalfilms are described, for example, in U.S. Pat. No. 6,924,014 (Ouderkirket al.) and also in PCT Publication WO2008/144636. Also useful aspartial reflectors are perforated partial reflectors or mirrors, suchas, for example, perforating ESR (available from 3M Company).

In one particular embodiment, substrate 610 can be a reflectivepolarizer. A reflective polarizer layer substantially reflects lightthat has a first polarization state and substantially transmits lightthat has a second polarization state, where the two polarization statesare mutually orthogonal. For example, the average reflectance of areflective polarizer in the visible for the polarization state that issubstantially reflected by the reflective polarizer is at least about50%, or at least about 60%, or at least about 70%, or at least about80%, or at least about 90%, or at least about 95%. As another example,the average transmittance of a reflective polarizer in the visible forthe polarization state that is substantially transmitted by thereflective polarizer is at least about 50%, or at least about 60%, or atleast about 70%, or at least about 80%, or at least about 90%, or atleast about 95%, or at least about 97%, or at least about 98%, or atleast about 99%. In some cases, the reflective polarizer substantiallyreflects light having a first linear polarization state (for example,along the x-direction) and substantially transmits light having a secondlinear polarization state (for example, along the z-direction).

Any suitable type of reflective polarizer may be used such as, forexample, a multilayer optical film (MOF) reflective polarizer such asVikuiti™ Dual Brightness Enhancement Film (DBEF), a diffusely reflectivepolarizing film (DRPF) having a continuous phase and a disperse phase,such as a Vikuiti™ Diffuse Reflective Polarizer Film (“DRPF”) availablefrom 3M Company, St. Paul, Minn., a wire grid reflective polarizerdescribed in, for example, U.S. Pat. No. 6,719,426, or a cholestericreflective polarizer.

For example, in some cases, the reflective polarizer layer can be orinclude an MOF reflective polarizer, formed of alternating layers ofdifferent polymer materials, where one of the sets of alternating layersis formed of a birefringent material, where the refractive indices ofthe different materials are matched for light polarized in one linearpolarization state and unmatched for light in the orthogonal linearpolarization state. In such cases, an incident light in the matchedpolarization state is substantially transmitted through the reflectivepolarizer and an incident light in the unmatched polarization state issubstantially reflected by reflective polarizer. In some cases, an MOFreflective polarizer can include a stack of inorganic dielectric layers.

As another example, the reflective polarizer can be or include apartially reflecting layer that has an intermediate on-axis averagereflectance in the pass state. For example, the partially reflectinglayer can have an on-axis average reflectance of at least about 90% forvisible light polarized in a first plane, such as the xy-plane, and anon-axis average reflectance in a range from about 25% to about 90% forvisible light polarized in a second plane, such as the xz-plane,perpendicular to the first plane. Such partially reflecting layers aredescribed in, for example, U.S. Patent Publication No. 2008/064133, thedisclosure of which is incorporated herein in its entirety by reference.

In some cases, the reflective polarizer can be or include a circularreflective polarizer, where light circularly polarized in one sense,which may be the clockwise or counterclockwise sense (also referred toas right or left circular polarization), is preferentially transmittedand light polarized in the opposite sense is preferentially reflected.One type of circular polarizer includes a cholesteric liquid crystalpolarizer.

In some cases, the reflective polarizer can be a multilayer optical filmthat reflects or transmits light by optical interference, such as thosedescribed in U.S. Publication 2011/0222263; U.S. Pat. Nos. 8,988,776;8,917,448; 8,662,687 and PCT Publication WO2008/144,136, allincorporated herein by reference in their entirety.

In one particular embodiment, substrate 610 can be a microstructuredsurface, such as a prismatic light directing film. For example, gradientoptical film 630 can be coated on the prism side of a light redirectingfilm such as Vikuiti™ Brightness Enhancing Film (BEF), available from 3MCompany. The BEF includes a plurality of linear prisms with, forexample, a 24 micron pitch and a prism peak or apex angle of about 90degrees. The gradient optical film 630 can be coated on themicrostructured surface as a conformal coating, a planarized coating, orpattern coated, as known to those of skill in the art.

Substantial portions of each two neighboring major surfaces in opticalconstruction 600 are in physical contact with each other along thebottom major surface 634 of gradient optical film 630. For example, atleast 50%, or at least 60%, or at least 70%, or at least 80%, or atleast 90%, or at least 95% of the two neighboring major surfaces are inphysical contact with each other. For example, in some cases, gradientoptical film 630 is coated directly on substrate 610.

FIG. 3 is a schematic side-view of an optical construction 700 thatincludes a gradient optical film 730 disposed on a substrate 710, and anoptical adhesive layer 720 disposed on gradient optical film 730.Substrate 710 can be any of the substrates described elsewhere,including, for example, a substrate such as substrate 610 described withreference to FIG. 2. In some cases the optical adhesive layer 720 canact as a sealer to inhibit infiltration of voids of gradient opticalfilm 730. In some cases, it may be desirable to have optical adhesivelayer 720 and gradient optical film 730 on opposite sides of thesubstrate 710. In other cases, it may be desirable to have gradientoptical film 730 on both sides of substrate 710.

Optical adhesive layer 720 can be any optical adhesive that may bedesirable and/or available in an application. Optical adhesive layer 720is of sufficient optical quality and light stability such that, forexample, the adhesive layer does not yellow with time or upon exposureto weather so as to degrade the optical performance of the adhesive andthe gradient optical film. In some cases, optical adhesive layer 720 canbe a substantially clear optical adhesive meaning that the adhesivelayer has a high specular transmittance and a low diffuse transmittance.For example, in such cases, the specular transmittance of opticaladhesive layer 720 is not less than about 70%, or not less than about80%, or not less than about 90%, or not less than about 95%.

In some cases, optical adhesive layer 720 is substantially opticallydiffusive, meaning that the adhesive layer has a high diffusetransmittance and a low specular transmittance, and the optical adhesivelayer 720 can have a white appearance. For example, in such cases, theoptical haze of an optically diffusive adhesive layer 720 is not lessthan about 30%, or not less than about 30%, or not less than about 50%,or not less than about 60%, or not less than about 70%, or not less thanabout 80%, or not less than about 90%, or not less than about 95%. Insome case, the diffuse reflectance of the diffusive adhesive layer isnot less than about 20%, or not less than about 30%, or not less thanabout 40%, or not less than about 50%, or not less than about 60%. Insuch cases, the adhesive layer can be optically diffusive by including aplurality of particles dispersed in an optical adhesive where theparticles and the optical adhesive have different indices of refraction.The mismatch between the two indices of refraction can result in lightscattering.

Exemplary optical adhesives include pressure sensitive adhesives (PSAs),heat-sensitive adhesives, solvent-volatile adhesives, repositionableadhesives or reworkable adhesives, and UV-curable adhesives such asUV-curable optical adhesives available from Norland Products, Inc.

Exemplary PSAs include those based on natural rubbers, syntheticrubbers, styrene block copolymers, (meth)acrylic block copolymers,polyvinyl ethers, polyolefins, and poly(meth)acrylates. As used herein,(meth)acrylic (or acrylate) refers to both acrylic and methacrylicspecies. Other exemplary PSAs include (meth)acrylates, rubbers,thermoplastic elastomers, silicones, urethanes, and combinationsthereof. In some cases, the PSA is based on a (meth)acrylic PSA or atleast one poly(meth)acrylate. Exemplary silicone PSAs include a polymeror gum and an optional tackifying resin. Other exemplary silicone PSAsinclude a polydiorganosiloxane polyoxamide and an optional tackifier.

Gradient optical film 730 can be similar to any gradient optical filmdisclosed herein. For example, gradient optical film 730 can be similarto one of gradient optical films 300A-300I. As another example, gradientoptical film 730 can include multiple layers, where each layer issimilar to one of gradient optical films 300A-300I.

In one particular embodiment, an optional optical diffuser (not shown)can be disposed on the optical adhesive layer 720, forming a stack ofoptical diffuser/optical adhesive/gradient optical film/substrate. Theoptional optical diffuser can include any optical diffuser that may bedesirable and/or available in an application. For example, the opticaldiffuser can be or include a surface diffuser, a volume diffuser, or acombination thereof. For example, the optional optical diffuser caninclude a plurality of particles having a first index of refraction n₁dispersed in a binder or host medium having a different index ofrefraction n₂, where the difference between the two indices ofrefraction is at least about 0.01, or at least about 0.02, or at leastabout 0.03, or at least about 0.04, or at least about 0.05.

FIG. 4 is a schematic side-view of an optical construction 800 thatincludes a first optical adhesive layer 820 disposed on a substrate 810,a gradient optical film 830 disposed on first optical adhesive layer820, and an optional second optical adhesive layer 840 disposed ongradient optical film 830. Substrate 810 can be any of the substratesdescribed elsewhere, including, for example, a substrate such assubstrate 610 described with reference to FIG. 2. Optical adhesivelayers 820 and 840 can be similar to optical adhesive layer 720. In somecases, optical adhesive layers 820 and 840 have the same index ofrefraction. In some cases, the two adhesive layers can have differentindices of refraction.

Gradient optical film 830 can be similar to any gradient optical filmdisclosed herein. For example, gradient optical film 830 can be similarto one of gradient optical films 300A-300I. As another example, gradientoptical film 830 can include multiple layers, where each layer issimilar to one of gradient optical films 300A-300I.

FIG. 8 is a schematic cross-section of a patterned retroreflector 900according to one aspect of the disclosure. Patterned retroreflector 900includes a substrate 910 having an array of cube-corner retroreflectors920. A first region 930 includes a high index material adjacent to thecube-corner retroreflectors 920. A second region 940 that includes a lowindex material adjacent to the cube-corner retroreflectors 920, isadjacent to the first region 930. The first and second regions 930, 940,are polymer gradient films that can be prepared and patterned accordingto the methods described elsewhere. A pigmented coating 950 can bedisposed over the first region 930 and the second region 940. A firstlight ray 960 incident on the cube-corner retroreflectors 920 adjacentthe second region 940, retroreflects as shown. A second light ray 970incident on the cube-corner retroreflectors adjacent the first region930 does not retroreflect, and instead shows the pigmented coating 950adjacent the first region 930.

FIG. 9 is a schematic cross-section of a patterned light guide 1000,according to one aspect of the disclosure. Patterned light guide 1000includes a light source 1010 capable of injecting light into a lightguide 1020. Light guide 1020 can be a hollow light guide or a solidlight guide, as described elsewhere. Light guide 1020 includes agradient polymer film 1030 that can be prepared as described elsewhere,such that a first region 1040 includes a lower index material than theindex of the light guide 1020, and a second adjacent region 1050includes a material having an index that is not lower than the index ofthe light guide 1020. Since light guide 1020 enables propagation oflight by TIR, a first light ray 1060 is shown to undergo TIR adjacentthe first region 1040, and a second light ray 1070 is shown to bedirected out of the light guide, since TIR 15 frustrated in the secondregion 1050. In some cases, the index of the second adjacent region 1050can be essentially the same as the index of the light guide 1020, andsecond light ray 1070 will exit the light guide without a change indirection, as known to one of skill in the art. Various extractorelements can be positioned adjacent the top surface 1080 of the gradientpolymer film 1030 to direct the light that escapes the light guide, asknown to one of skill in the art.

In some cases, the disclosed gradients can be combined in any fashiondesired, to create X (downweb), Y (crossweb), XY, XZ, YZ, and XYZgradient combinations. They can also be combined with any desiredsurface patterning, and applied to different substrates including, forexample, PET, Polycarbonate, MOF, microreplicated optical films, and thelike.

In some cases, substrate can be at least one of a release liner, anadhesive, a volume diffuser, a surface diffuser, a diffractive diffuser,a refractive diffuser, a retroreflector, an absorbing polarizer, areflective polarizer, a fiber polarizer, a cholesteric polarizer, amultilayer polarizer, a wire grid polarizer, a partial reflector, avolume reflector, a multilayer polymer reflector, a metal reflector, ametal/dielectric multilayer reflector, a fiber, a lens, amicrostructure, a solid light guide, or a hollow light guide. In somecases, the microstructure can be a retroreflector, a brightnessenhancing film (BEF), a lenslet, a gain diffuser, a light extractionfilm, or a turning film.

In some cases, a gradient polymeric film can include multiple layers,where one or more layers is similar to one of gradient optical films300A-300I, one or more layers includes a “z” gradient film as describedelsewhere, or one or more layers includes a non-gradient film, or acombination of gradient films and non-gradient films. In some cases, thegradient polymeric film can include a combination of layers that are lowhaze/high haze/low haze. Generally, such other layers in a multilayercoating can include, for example, a volume diffuser, porous coatings,diffuse porous coatings, sealants, primer, adhesives and the like. Themultilayer coating layer can be the surface layer of a subsurface layerof a multilayer coating stack. Generally, a multilayer coating can beproduced either simultaneously or sequentially, as known to one of skillin the art.

In one particular embodiment, a graded optical film having a patterneddifference in the refractive index can be useful for light extraction.Such a graded optical film can enable selective transmission from, forexample, a light guide to a light re-directing element. A lightguidepropagates light across an area due to total internal reflection (TIR)from the surfaces of the lightguide. TIR occurs where there is a largeindex difference from the guide to the surrounding medium. A gradientoptical film having regions of low index alternating with regions ofsimilar index that is laminated to the light guide, can cause light toundergo selective TIR where the index is low, but light can be allowedto escape the guide where the higher index region is. Typically, thiscontrolled extraction can be used to meter the light to a lightre-directing element including prisms, gain diffusers, turning films, orother such structure as known in the art. In some cases, lightre-directing elements could either be microreplicated, or an angleselective MOF.

The disclosed gradient films can be used in applications including, forexample, light guide variable extractors including solid light guideextractors, hollow (air) guide extractors, fibers and the like; gradienthaze films useful, for example, for defect and/or bulb hiding,particularly in backlit displays; variable diffusers; variableabsorbers; variable reflectors including enhanced specular reflectors(ESR) for daylighting; and the like.

EXAMPLES

In the examples that follow, the transmission, haze, and clarity weremeasured using a BYK-Gardner Haze-Gard Plus haze meter (available fromBYK-Gardner, Silver Springs, Md.). Unless otherwise specified, allchemicals are available from Aldrich Chemical, Milwaukee, Wis. Therefractive index (RI) of the coating was measured using a Model 2010Prism Coupler (available from Metricon Corporation, Pennington N.J.).The Model 2010 Metricon was configured with a HeNe laser operating at awavelength of 632.8 nm and an optical prism (code 6567.9). Themeasurements were made in both the TE and TM modes. To determine thefilm side refractive index of the coating, the sample was loaded suchthat the substrate was in intimate contact with the prism coupler. Todetermine the air side refractive index of the coating, the sample wasloaded such that the coating was in intimate contact with the prismcoupler.

FIG. 5 shows a schematic of a process 200 used to program and controlthe lamps (in this case, the UV LEDs) to generate, for example, atemporal gradient, according to one aspect of the disclosure. Process200 includes a first step 210 that generated an Amps vs % Haze (or otherdesired control curve, such as, for example, % T, % C, or refractiveindex) curve, a second step 220 that generated a control voltage vs Ampscurve, and a third step 230 that converted Amps to control voltage toresult in the control voltage vs % Haze curve. As a result, in steps210-230 shown in FIG. 5, the voltage vs haze (or alternately voltage vs% T) curve was created by creating samples of high haze GEL at controllamp voltages from 0V to 10V in steps of 0.5V. Process 200 furtherincludes a fourth step 240 in which the required % Haze gradient wasdefined, a fifth step 250 that fitted a function to the required % Hazegradient, and a sixth step 260 that interpolated the desired gradient toobtain the % Haze at a short time interval, for example, about 0.1seconds. As a result, in steps 240-260, the required haze gradient wasdefined based on the final intended use of the gradient. The requiredcurve was then fit with a polynomial describing the curve in Matlab(available from The MathWorks, Natick, Mass.). This function was thenused to interpolate points between the required haze gradient curve atintervals of 0.001 inches (25.4 microns). Second, third, fifth, sixth,and seventh steps 220, 230, 250, 260, and 270 in process 200 typicallyreside in a software program 290. Process 200 still further includes aseventh step 270 that combined the control voltage vs % Haze curve fromthird step 230 with the interpolated desired gradient that provided the% Haze at short time intervals from sixth step 260. As a result, inseventh step 270, the control voltage vs haze curve was then used tocalculate the voltage required at each position to obtain the requiredhaze value. Process 200 further includes an eighth step 280 that appliedthe required voltage gradient to the lamps, using the result fromseventh step 270. As a result, in eighth step 280, the voltage curve wasapplied to the lamps during the creation of the sample.

Preparation of Coating Solution “A”

Nalco 2327 (400 g) (20 nm colloidal silica dispersion available fromNalco, Naperville Ill.) was charged to a 1 qt jar. 1-methoxy-2-propanol(450 g), trimethoxy(2,4,4-trimethylpentyl)silane (11.95 g) (availablefrom Waker Silicones Adrian Mich.), 4-(Triethoxysilyl)-butyronitrile(11.85 g) and 5% Prostab 5128 in water (0.23 g) (available from CibaSpecialties Chemical, Inc Tarrytown, N.Y.) were mixed together and addedto the colloidal dispersion while stirring. The jar was sealed andheated to 80 C for 16 hr.

The resulting solution was allowed to cool down to room temperature. Theabove dispersion (606.7 g) and 1-methoxy-2-propanol (102.3 g) werecharged to a 1000 ml RB flask. Water and 1-methoxy-2-propanol wereremoved via rotary evaporation to a weight of 314.8 g. Additionaldispersion (258.61 g) and 1-methoxy-2-propanol (202.0 g) were charged tothe flask. Water and 1-methoxy-2-propanol were removed via rotaryevaporation to give a weight of 343.69 g. 1-methoxy-2-propanol (89.2 g)was added to give an approximately 43 wt % solids dispersion of surfacemodified 20 nm silica in 1-methoxy-2-propanol.

The resulting solution was 43% wt modified 20 nm silica dispersed in1-methoxy-2-propanol. Next, 100 g of this solution, 64.5 g of SR 444(available from Sartomer Company, Exton Pa.), 2.15 g of photoinitiatorIrgacure 184 (available from Ciba Specialty Chemicals Company, HighPoint N.C.), and 167.2 g of isopropyl alcohol and 26.6 g of1-methoxy-2-propanol were mixed together by stirring to form ahomogenous coating solution A (30% solids coating solution).

Preparation of Coating Solution “B”

A coating solution “B” was made. First, 360 g of Nalco 2327 colloidalsilica particles (40% wt solid and an average particle diameter of about20 nanometers) (available from Nalco Chemical Company, Naperville Ill.)and 300 g of solvent 1-methoxy-2-propanol were mixed together underrapid stirring in a 2-liter three-neck flask that was equipped with acondenser and a thermometer. Next, 22.15 g of Silquest A-174 silane(available from GE Advanced Materials, Wilton Conn.) was added. Themixture was stirred for 10 min. Next, an additional 400 g of1-methoxy-2-propanol was added. The mixture was heated at 85° C. for 6hours using a heating mantle. The resulting solution was allowed to cooldown to room temperature. Next, most of water and 1-methoxy-2-propanolsolvents (about 700 g) were removed using a rotary evaporator under a60° C. water-bath.

The resulting solution was 43% wt A-174 modified 20 nm silica cleardispersed in 1-methoxy-2-propanol. Next, 82.65 g of this solution, 24 gof SR 444 (available from Sartomer Company, Exton Pa.), 0.119 g ofphotoinitiator Irgacure 819 (available from Ciba Specialty ChemicalsCompany, High Point N.C.), and 91.7 g of isopropyl alcohol were mixedtogether by stirring to form a homogenous coating solution B (30% solidscoating solution).

Example 1: Downweb Gradients of Haze and Transmission

Example 1 demonstrates downweb gradients of haze while holding thepercent transmission at a constant value, and downweb gradients oftransmission while holding haze constant.

Generation of Calibration Curve:

The coating solution “A” was syringe-pumped at a rate of 2.5 cc/min intoa 10.15 cm (4-inch) wide slot-type coating die. The slot coating dieuniformly distributed a 10.15 cm wide coating onto a PET substratemoving at 5 ft/min (152 cm/min).

The coating was then polymerized by passing the coated substrate througha UV-LED cure chamber that included a quartz window to allow passage ofUV radiation. The UV-LED bank included a rectangular array of 352UV-LEDs, 16 down-web by 22 crossweb (approximately covering a 20.3cm×20.3 cm area). The UV-LEDs were placed on two water-cooled heatsinks. The LEDs (available from Cree, Inc., Durham N.C.) operated at anominal wavelength of 395 nm, and were run at 45 Volts at 13 Amps. TheUV-LED array was powered and fan-cooled by a TENMA 72-6910 (42V/10A)power supply (available from Tenma, Springboro Ohio). The UV-LEDs werepositioned above the cure chamber quartz window at a distance ofapproximately 2.54 cm from the substrate. The UV-LED cure chamber wassupplied with a flow of nitrogen at a flow rate of 46.7 liters/min (100cfh) resulting in an oxygen concentration of approximately 150 ppm inthe cure chamber. The oxygen concentration in all cases was measuredusing a sensor located below the quartz window in the cure chamber, inthe center of the coated width at a distance of approximately ¼″ (6.4mm) from the coating.

After being polymerized by the UV-LEDs, the solvent in the cured coatingwas removed by transporting the coating to a drying oven operating at150° F. (66 C) for 2 minutes at a web speed of 5 ft/min. Next, the driedcoating was post-cured using a Fusion System Model 1300P configured withan H-bulb (available from Fusion UV Systems, Gaithersburg Md.), operatedat full power. The UV Fusion chamber was supplied with a flow ofnitrogen that resulted in an oxygen concentration of approximately 50ppm in the chamber.

The power supply was controlled by applying a control voltage to thepower supply's input pin with a Compaq 6910p laptop and a DAQCard-6062EPCMCIA Multifunction I/O card (National Instruments, Austin, Tex.)controlled with LabView software (National Instruments, Austin Tex.).Samples were created with control voltages ranging from 0 to 10 V insteps of 0.5 volts, these voltages corresponding to output amps at thelamp of from 0 to 12 Amps. The percent transmission (% T), haze (% H)and clarity (% C) were measured on a BYK-Gardner Haze-gard plus, andshown plotted vs. dose in FIGS. 6A-6B, along with the fitting curves aspreviously described with reference to FIG. 5.

Generation of Haze Gradient Sample:

The same coating solution as used for the calibration curve, above, wassyringe-pumped at a rate of 5 cc/min into a 20.3 cm (8-inch) wideslot-type coating die. The slot coating die uniformly distributed a 20.3cm wide coating onto a PET substrate moving at 5 ft/min (152 cm/min).

The sample was processed in the same technique as the calibration curve.Samples were created by applying the temporal control voltage ramp tothe lamp power supply as the web was passing through the lamp cureregion. The voltage ramp profile is shown in FIG. 6C. The corresponding% H and % T vs position is shown in FIG. 6D. The graph shows that theHaze changes linearly with position from about 10% H to about 95% H,while the % T remains constant throughout the length of the sample.

Generation of Transmission Gradient Sample:

The same coating solution as used for the calibration curve above wassyringe-pumped at a rate of 5 cc/min into a 20.3 cm (8-inch) wideslot-type coating die. The slot coating die uniformly distributed a 20.3cm wide coating onto a PET substrate moving at 5 ft/min (152 cm/min).

The sample was then processed using the same technique as thecalibration curve. Samples were created by applying the temporal controlvoltage ramp to the lamp power supply as the web was passing through thelamp cure region. The voltage ramp profile is shown in FIG. 6E. Thecorresponding % H and % T vs position is shown in FIG. 6F. The graphshows that the Transmission varies from about 60-80% T while the % Hremains essentially constant throughout the length of the sample.

Example 2: Crossweb Gradients of Haze and Transmission

The coating solution “A” was syringe-pumped at a rate of 2.5 cc/min intoa 10.15 cm (4-inch) wide slot-type coating die. The slot coating dieuniformly distributed a 10.15 cm wide coating onto a PET substratemoving at 5 ft/min (152 cm/min).

The coating was then polymerized by passing the coated substrate througha UV-LED cure chamber that included a quartz window to allow passage ofUV radiation. The UV-LED bank included a rectangular array of 352UV-LEDs, 16 down-web by 22 crossweb (approximately covering a 20.3cm×20.3 cm area). The UV-LEDs were placed on two water-cooled heatsinks. The LEDs (available from Cree, Inc., Durham N.C.) operated at anominal wavelength of 395 nm, and were run at 45 Volts at 13 Amps. TheUV-LED array was powered and fan-cooled by a TENMA 72-6910 (42V/10A)power supply (available from Tenma, Springboro Ohio). The UV-LEDs werepositioned above the cure chamber quartz window at a distance ofapproximately 2.54 cm from the substrate. The UV-LED cure chamber wassupplied with a flow of nitrogen at a flow rate of 46.7 liters/min (100cubic feet per hour) resulting in an oxygen concentration ofapproximately 150 ppm in the cure chamber. A chrome-on-quartz mask waspositioned between the LED lamps and the coating. This mask was a lineargradient of transmission ranging from 100% T to 0% T across the 10.15 cmcoating width.

After being polymerized by the UV-LEDs, the solvent in the cured coatingwas removed by transporting the coating to a drying oven operating at150° F. (66 C) for 2 minutes at a web speed of 5 ft/min. Next, the driedcoating was post-cured using a Fusion System Model I300P configured withan H-bulb (available from Fusion UV Systems, Gaithersburg Md.), operatedat full power. The UV Fusion chamber was supplied with a flow ofnitrogen that resulted in an oxygen concentration of approximately 50ppm in the chamber.

The resulting sample had a high haze with low transmission along oneedge, and a low haze with high transmission along an opposite edge.

Example 3: Combined Downweb and Thickness (Z-Axis) Gradients ofRefractive Index

The coating solution “B” was syringe-pumped at a rate of 2.5 cc/min intoa 10.15 cm (4-inch) wide slot-type coating die. The slot coating dieuniformly distributed a 10.15 cm wide coating onto a PET substratemoving at 5 ft/min (152 cm/min).

The coating was then polymerized by passing the coated substrate througha UV-LED cure chamber that included a quartz window to allow passage ofUV radiation. The UV-LED bank included a rectangular array of 352UV-LEDs, 16 down-web by 22 crossweb (approximately covering a 20.3cm×20.3 cm area). The UV-LEDs were placed on two water-cooled heatsinks. The LEDs (available from Cree, Inc., Durham N.C.) operated at anominal wavelength of 395 nm, and were run at 45 Volts at 13 Amps. TheUV-LED array was powered and fan-cooled by a TENMA 72-6910 (42V/10A)power supply (available from Tenma, Springboro Ohio). The UV-LEDs werepositioned above the cure chamber quartz window at a distance ofapproximately 2.54 cm from the substrate. The UV-LED cure chamber wassupplied with a flow of nitrogen at a flow rate of 46.7 liters/min (100cubic feet per hour). A 1.5 cfh (0.7 liters/min) stream of air was bledinto this nitrogen stream resulting in an oxygen concentration at thelamp of approximately 1000 ppm. The high concentration of oxygen in thecure chamber resulted in inhibited curing at the surface of the coating,as described, for example, in U.S. Pat. No. 9,279,918. After the coatingwas further dried and cured, according the technique described elsewherein Example 1, this inhibition resulted in a gradient of refractive indexthrough the thickness of the coating. As a result, a “skin” coatingwhich had very low porosity (and corresponding higher refractive index)developed on the surface closest to the oxygen inhibition.

Simultaneously, the power supply was temporally controlled, as describedelsewhere, by applying a control voltage to the power supply's input pinwith a Compaq 6910p laptop and a DAQCard-6062E PCMCIA Multifunction I/Ocard (National Instruments, Austin Tex.) controlled with LabViewsoftware (National Instruments, Austin Tex.). Samples were created byapplying the temporal control voltage ramp to the lamp power supply asthe web was running through the lamp. FIG. 7A shows the voltage rampprofile used in Example 3.

The refractive indices of each of the samples were measured in twoorientations: first with the coating side adjacent to the grating,second with the PET substrate adjacent to the prism, as describedelsewhere. The corresponding refractive index vs position is shown inFIG. 7B. First curve 410 shows the refractive index of the low indexlayer at the surface of the layer. Curve 410 shows that the index variesfrom 1.38 to 1.48. Second curve 420 shows the index of the low indexlayer in the area adjacent to the interface between the substrate andthe low index coating. At each position there is a difference in indexbetween the interior of the coating shown by second curve 420 and thetop of the coating shown by first curve 410. This difference at eachposition defines the ‘Z’ gradient of the coating.

Second curve 420 shows the refractive index in the interior of thecoating varies from 1.22 to 1.48. This variation of index with positiondefines the ‘X’ gradient of the coating. Third curve 430 shows the indexof the PET substrate is constant throughout the sample.

Example 4: Bulb-Hiding Gradient for Direct-Lit Backlights

This Example describes a film that was produced having haze gradientsspecifically designed to increase bulb hiding in direct-lit backlights,such as used in liquid crystal displays. The haze gradient film wascharacterized by having stripes of high haze and low haze areas,positioned at the pitch of the bulbs. When the high haze stripes werealigned over the bulb, they can even out the bright spots of incidentlight coming from the bulb. The pattern of the haze was created byoptically patterning the porous layer made by the GEL process usingphotomasks under the LED lamps, and demonstrates patterned morphologyusing the GEL process.

Preparation of Coating Solution “C”

N-(3-triethoxysilylpropyl)methoxyethoxyethyl carbamate (PEG2silane) wasfirst prepared. A 250 ml round-bottomed flask equipped with a magneticstir bar was charged with diethylene glycol methyl ether (150 g) andmethyl ethyl ketone (65 g). A majority of the solvent was removed viarotary evaporation to remove water. 3-(triethoxysilyl)propylisocyanate(308.5 g) was charged to the flask. Dibutyltin dilaurate (˜3 mg) wasadded, and the mixture stirred. The reaction proceeded with a mildexotherm. The reaction was run for approximately 16 hr at which timeinfra red spectroscopy showed no isocyanate was left. The remainder ofthe solvent was removed via rotary evaporation (90 C). The resultingPEG2silane was a clear colorless liquid.

In a 2 liter three-neck flask, equipped with a condenser and athermometer, 288 grams of Nalco 2327 (40% wt 20 nm slica dispersed inwater, available from Nalco, Naperville, Ill.) and 300 g of1-methoxy-propanol were mixed together under rapid stirring. After that,8.35 g of trimethoxy(2,4,4 trimethylpentyl) silane (available fromGelest, Morrisville, Pa.) and 13.12 g of PEG2 silane (described above)were added, then the mixture was stirred for 30 min. 500 g of additional1-methoxy-propanol was then added. The mixture was heated to 85° C. for6 hours. The resulting solution was allowed to cool down to roomtemperature. Most of the solvents of water/1-methoxy-propanol wereremoved using a rotary evaporator in a water-bath at 60° C., resultingin a 42.87% wt isooctyl/PEG2 modified 20 nm silica solution. The processwas repeated several times to result in a large batch for processing.

Coating solution C was prepared by mixing the following together underrapid stirring until a homogenous coating solution obtained: 292.5 gramsof 42.87% isooctyl/PEG2 modified 20 nm silica solution, 153.6 gramsSR444 (available from Sartomer, Exton, Pa.), 400 grams isopropylalcohol, 30 grams 1-methoxy-propanol, and 8.5 grams Irgacure 184 (CibaSpecialties Chemical, Tarrytown, N.Y.).

Coating solution “C” was syringe-pumped at a rate of 5.0 cc/min into a20.3 cm (8-inch) wide slot-type coating die. The slot coating dieuniformly distributed a 20.3 cm wide coating onto a substrate moving at5 ft/min (152 cm/min).

Next, the coating was polymerized by passing the coated substratethrough a UV-LED cure chamber that included a quartz window to allowpassage of UV radiation. The UV-LED bank included a rectangular array of352 UV-LEDs, 16 down-web by 22 crossweb (approximately covering a 20.3cm×20.3 cm area). The UV-LEDs were placed on two water-cooled heatsinks. The LEDs (available from Cree, Inc., Durham N.C.) operated at anominal wavelength of 395 nm, and were run at 45 Volts at 13 Amps. TheUV-LED array was powered and fan-cooled by a TENMA 72-6910 (42V/10A)power supply (available from Tenma, Springboro Ohio). The UV-LEDs werepositioned above the cure chamber quartz window at a distance ofapproximately 2.54 cm from the substrate. The UV-LED cure chamber wassupplied with a flow of nitrogen at a flow rate of 46.7 liters/min (100cubic feet per hour) resulting in an oxygen concentration ofapproximately 150 ppm in the cure chamber. A photomask was alignedunderneath the UV-LEDs, between the UV-LED lamps and the (20.3 cm×20.3)quartz plate. The photomask was a PET substrate covered with aluminumtape trapezoids arranged in a linear crossweb pattern to block a portionof the light from the coated web.

After being polymerized by the UV-LEDs, the solvent in the cured coatingwas removed by transporting the coating to a drying oven operating at150° F. for 2 minutes at a web speed of 5 ft/min. Next, the driedcoating was post-cured using a Fusion System Model 1300P configured withan H-bulb (available from Fusion UV Systems, Gaithersburg Md.). The UVFusion chamber was supplied with a flow of nitrogen that resulted in anoxygen concentration of approximately 50 ppm in the chamber. Asinusoidal variation in the % T and % C resulted, where the % T and % Cranged from 76% T and 72% C in a first region, to 58% T and 0.6% C in asecond adjacent region.

Example 5 Patterned Retroreflective Film

This Example describes a patterned retroreflector that includes areasthat are retroreflective and areas that are not retroreflective. Theposition of these areas was controlled by the refractive index of thematerial behind a corner cube retroreflector. The pattern of the indexwas created by optically patterning the porous layer made by the GELprocess and demonstrates patterned morphology using the GEL process.

Preparation of Coating Solution “D”

In a 2 liter three-neck flask, equipped with a condenser and athermometer, 960 grams of IPA-ST-UP organosilica elongated particles(15.6% wt elongated silica dispersed in isopropyl alcohol, availablefrom Nissan Chemical America, Houston, Tex.), 19.2 grams of deionizedwater, and 350 grams of 1-methoxy-2-propanol were mixed under rapidstirring. The elongated particles had a diameter in a range from about 9nm to about 15 nm and a length in a range of about 40 nm to about 100nm. The particles were dispersed in a 15.2% wt IPA, and 22.8 grams ofSilquest A-174 silane (available from GE Advanced Materials, WiltonConn.) was added to the flask. The resulting mixture was stirred for 30minutes.

The mixture was kept at 81° C. for 16 hours. Next, the solution wasallowed to cool down to room temperature, and about 950 grams of thesolvent in the solution was removed using a rotary evaporator under a40° C. water-bath, resulting in a clear A-174-modified elongated silicasolution having 44.56% wt A-174-modified elongated silica dispersed in1-methoxy-2-propanol (herein A-174 modified UP silica). The process wasrepeated several times to result in a large batch for processing.

Coating solution “D” was prepared by mixing the following together underrapid stirring until a homogenous coating solution was obtained: 336.8grams A-174 modified UP silica, 150 grams SR444 (available fromSartomer, Exton, Pa.), 263 grams isopropyl alcohol, 7.5 grams Irgacure184, and 0.375 grams Irgacure 819 (both available from Ciba SpecialtiesChemical, Tarrytown, N.Y.).

The cube-corner side of a cube-corner retroreflector was hand coatedwith Coating Solution “D”, and a polypropylene release liner was placeover the coating. A nickel patterned on quartz mask having a rectangulargrid pattern was placed on top of the polypropylene release liner. Thesample was then cured in a belt fed cure chamber (RPC industries) fittedwith a Fusion H bulb in air. The sample was then removed from thechamber, the mask and release liner removed, and the sample was placedin a 120 F oven for about 5 minutes to dry. The sample was then passedagain through the cure chamber (in a nitrogen atmosphere) to fully curethe remaining acrylate. The sample was hand laminated with TiO2 loadedtransfer adhesive. The samples showed the rectangular grid pattern wasvisible in retroreflection, where the low index coating retained theoptical activity of the retroreflector.

The embodiments described can be used anywhere that an optical film canbe used, for example, for control of optical properties of refractiveindex, haze, transmission, and clarity. In general, the embodimentsdescribed can be applied anywhere that thin, optically transmissivestructures are used, including light management films or lightmanagement film stacks; backlights including hollow and solidbacklights; displays such as TV, notebook computers, computer monitors;and also used for advertising, information display or lighting. Thepresent disclosure is also applicable to electronic devices includinglaptop computers and handheld devices such as Personal Data Assistants(PDAs), personal gaming devices, cell phones, personal media players,handheld computers and the like, which incorporate optical displays.Backlights using textured films of the present disclosure haveapplication in many other areas. For example, backlit LCD systems,luminaires, task lights, light sources, signs and point of purchasedisplays can be made using the described embodiments.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe foregoing specification and attached claims are approximations thatcan vary depending upon the desired properties sought to be obtained bythose skilled in the art utilizing the teachings disclosed herein.

All references and publications cited herein are expressly incorporatedherein by reference in their entirety into this disclosure, except tothe extent they may directly contradict this disclosure. Althoughspecific embodiments have been illustrated and described herein, it willbe appreciated by those of ordinary skill in the art that a variety ofalternate and/or equivalent implementations can be substituted for thespecific embodiments shown and described without departing from thescope of the present disclosure. This application is intended to coverany adaptations or variations of the specific embodiments discussedherein. Therefore, it is intended that this disclosure be limited onlyby the claims and the equivalents thereof.

What is claimed is:
 1. A gradient polymer film, comprising: a binder;and a plurality of nanovoids, wherein a local volume fraction of theplurality of nanovoids varies across a transverse plane of the gradientpolymer film and varies along a first direction and a second directionorthogonal to the first direction, across the transverse plane.
 2. Thegradient polymer film of claim 1, wherein the local volume fractionvaries in a pattern across the transverse plane.
 3. An opticalconstruction, comprising: a substrate; the gradient polymer film ofclaim 1 disposed on the substrate; and an optical adhesive layerdisposed on an opposing surface of the gradient polymer film or thesubstrate.
 4. The optical construction of claim 3 further comprising anoptical adhesive layer disposed between the substrate and the gradientpolymer film.
 5. A gradient polymer film, comprising: a binder; and aplurality of nanovoids, wherein a first local volume fraction of theplurality of nanovoids proximate a first region of the gradient polymerfilm is greater than a second local volume fraction of the plurality ofnanovoids proximate a second region adjacent the first region, along atransverse plane of the gradient polymer film, wherein the first localvolume fraction of the plurality of nanovoids decreases in a step-wisemanner to the second local volume fraction of the plurality ofnanovoids, along the transverse plane of the gradient polymer film. 6.An optical construction, comprising: a substrate; the gradient polymerfilm of claim 5 disposed on the substrate; and an optical adhesive layerdisposed on an opposing surface of the gradient polymer film or thesubstrate.
 7. The optical construction of claim 6 further comprising anoptical adhesive layer disposed between the substrate and the gradientpolymer film.